In powder metallurgy for producing a sintered body by using an iron powder or a copper powder as the main raw material, generally a mixed powder containing the powder of a main raw material, an auxiliary material powder (a graphite powder, an alloy component, etc.) for improving the physical property of the sintered body, a lubricant, and others is used. In order to improve the mechanical properties (strength, hardness, etc.) of a sintered body in particular, generally a means of adding a carbon supply component (carbon source) such as graphite, molding a material powder, and successively dispersing the carbon source in an iron powder and carburizing the iron powder during heat sintering process is adopted.
Since graphite has a smaller specific gravity and a smaller grain size than an iron powder however, a problem is that, only by mixing them, the graphite is largely separated from the iron powder, the graphite segregates, and they cannot be mixed homogeneously. In powder metallurgy, generally a mixed powder is stored in a storage hopper in advance for mass-producing sintered bodies. In a storage hopper, graphite having a small specific gravity tends to segregate at the upper part of the hopper and, when a mixed powder is discharged from the hopper, the concentration of the graphite increases toward the end of the discharge from the hopper. As a result, a part having a high carbon concentration is formed in a sintered body, a cementite structure precipitates there, and mechanical properties deteriorate. If a carbon content varies by the segregation of graphite in a sintered body, parts having a stable quality can hardly be produced. Further an arising problem in a mixing process and a molding process is that the segregated graphite powder causes dust emission, and the deterioration of a work environment and the lowering of the handleability of a mixed powder are caused. Such segregation is caused not only in the case of graphite but also in the cases of various kinds of powders mixed with an iron powder likewise and the prevention of the segregation is desired.
In order to prevent such segregation and dust emission, roughly three methods have heretofore been proposed. The first method is a method of adding a liquid additive such as tall oil to a mixed powder (for example, Patent Literatures 1 and 2). The method has an advantage that a mixed powder can be produced with simple equipment but a problem of the method is that, if a liquid additive of a quantity necessary for exhibiting a segregation prevention effect is added, a liquid bridge force acts among iron particles and flowability deteriorates extremely. The second method is a method of vaporizing a solvent and attaching graphite onto the surface of an iron powder after dissolving a solid binder such as a high molecular weight polymer into the solvent and homogenously mixing them (Patent Literatures 3 and 4 and others). The method has the advantages that graphite can stick without fail and there are many choices in adopting a lubricant used but the flowability of a mixed powder may be insufficient depending on some quantities or some types. The third method is a so-called hot melt method characterized by heating and melting a lubricant of a relatively low molecular weight such as fatty acid while it is mixed with an iron powder (for example, Patent Literature 5). The drawback of the method is that the temperature control during mixing is very important for uniformly sticking the melted lubricant onto the surface of the iron powder and the number of choices for a usable lubricant is restricted.
For preventing the segregation and dust emission of graphite, adhesive force between an iron powder and graphite is required to be enhanced but other characteristics are also required in recent years and the types and degrees of the characteristics have increasingly been upgraded. As one of the required characteristics, the flowability of powder is named. In powder metallurgy, the flowability of a mixed powder is one of the important characteristics when the mixed powder is discharged from a storage hopper or when a mold is filled with the mixed powder. That is, if the flowability of a mixed powder is inferior, the arising problems are that bridging is caused at the upper part of the outlet in a hopper, thus the discharge is hindered, and a hose clogs between the hopper and a shoebox. Further, in the case of a mixed powder having a poor flowability, even when the mixed powder is outpoured forcibly from a hose, a mold, particularly a part of thin-wall, is not filled and a sound molded body may not be produced in some cases.
The flowability of a mixed powder is influenced also by the grain size and shape of the metal powder used, the type, quantity, grain size, and shape of a physical property improving agent to be added, and others and the most influencing factors are considered to be the quantity of a powdery lubricant added and the type of a lubricant added to the mixed powder.
With regard to the quantity of an added powdery lubricant, generally flowability deteriorates from its peak at 0.1% by mass of the added lubricant as the quantity of the added lubricant increases and hence it is preferable to lower the quantity of the added lubricant from the viewpoint of securing the flowability. If the quantity of the added lubricant decreases however, the lubricity lowers considerably as a matter of course, the friction coefficient between a molded body and a mold face increases when the molded body is extracted from a mold, and that causes die seizure and mold damage. Consequently, it has been difficult to obtain both lubricity and flowability simultaneously.
Further, it is difficult to obtain both lubricity and flowability simultaneously also from the viewpoint of the type and melting point of a lubricant. That is, stearic acid or stearic acid amide generally having a low melting point is excellent in lubricity but, in a lubricant having such a low melting point, aggregation is caused and flowability deteriorates in some cases. When an ambient temperature is high in particular, the drawback appears conspicuously. In contrast, metallic soap or ethylene bis-amide having a high melting point can maintain a good flowability even when an ambient temperature rises but lubricity is inferior to the stearic acid amide or the like having a low melting point.
In this way, in consideration of the quantity and type of a lubricant added, to materialize a mixed powder having both lubricity and flowability simultaneously has been a long-term challenge.